1. Field of the Invention
The present invention relates generally to a method for programming a flash memory cell, and more particularly, to a method for programming a 2-bit charge-trapping nonvolatile memory cell using raised-VS channel Initialed secondary electron Injection (CHISEL) programming method.
2. Description of the Related Art
Widely used in semiconductor industry, charge-trapping nonvolatile memory cells are type of memory cells that store charges at its charge-trapping layer and retain the stored charges when power is remove. If a charge-trapping nonvolatile memory cell can store charges at both ends of the charge-trapping layer, this charge-trapping nonvolatile memory cell is capable of 2-bit operations.
A charge-trapping nonvolatile memory cell can be programmed by a traditional channel hot electron (CHE) programming method. However, the traditional CHE programming method has the drawbacks of large programming current and low programming speed. In order to overcome the shortcomings of the CHE programming method, a conventional channel Initialed secondary electron Injection (CHISEL) programming method is used to program a charge-trapping nonvolatile memory cell by grounding the source and applying a negative substrate voltage to enhance the secondary electron generation.
Although the conventional CHISEL programming method is more efficient than the traditional CHE programming method, the major drawback of the conventional CHISEL programming method is that a negative voltage needs to be applied to the substrate of the charge-trapping memory cell to be programmed. Since the substrate is highly capacitive and resistive for a large sector of the charge-trapping memory cells, the charging and discharging of the substrate for a charge-trapping nonvolatile memory cell will result in long response time and large power consumption.
In view of the foregoing, there is a need for an improved CHISEL programming method that will avoid the need of charging and discharging the highly capacitive and resistive substrate of a charge-trapping nonvolatile memory cell.